The present invention relates to a method of synchronizing a radio receiver, and a receiver suitable for implementing such a method. It also relates to a transmission method comprising such a synchronization of the receiver, and a suitable radio transmitter.
Pulse radio transmission is a recent remote communication technique, used mainly for short range applications. The transmission distance is typically less than 100 meters. It can cover transmissions at very high rate, at least 500 megabits/second for example, used in various circumstances such as, in particular, a USB2 port, a video data transmission between a DVD player and a projector, or even a data broadcast from an information station made available to the public. It can also cover transmissions at relatively low rate, for example for linking sensors to a monitoring or data acquisition local area network. In this case, the pulse transmission is particularly advantageous because of its low energy consumption.
In a pulse radio transmission, a predetermined transmission duration is assigned to each symbol. The term “symbol” is used to mean a modulation state used in coding data intended for transmission. For a 2-PPM (standing for “Pulse Position Modulation”) transmission mode, a symbol corresponds to one transmitted bit, and for a 4-PPM mode, a symbol corresponds to two transmitted bits. The symbol transmission duration is divided into a fixed number of first-level time slots, called frames. Furthermore, each frame is itself divided into second-level time slots, called slots. As an example, the transmission duration of a symbol (denoted TS) is 1280 ns (ns standing for nanoseconds), the duration of a frame (denoted TF) is 160 ns and that of a slot (denoted TC, standing for “chip” duration) is 20 ns. In this case, one symbol corresponds to eight frames transmitted in succession, and each frame in turn corresponds to eight slots.
A symbol is transmitted in the form of pulses located within certain slots of a transmission sequence. It is coded via a delay of each pulse relative to the start of the corresponding slot, called state delay. To resume the example mentioned above, a pulse duration of around 1 ns and a state delay also of around 1 ns are suitable for 20 ns slots. Because of the very short duration of the pulses, the transmitted radio energy is distributed over a very wide range of frequencies, of the order of several gigahertz. For this reason, the pulse radio transmission mode is designated UWB, standing for “Ultra-Wide Band”.
Within the transmission sequence of each symbol, different slot selections for each frame make it possible to obtain multiple access. In other words, several transmission channels are thus distinguished, which can be used simultaneously to transmit a variety of data to different receivers. The ordered series of position numbers, within successive frames, of the slots that contain a meaningful pulse to transmit a symbol via this channel constitutes a characteristic code of the channel. This code is therefore formed by a pseudo-random series of N values, N being the number of frames within the transmission sequence of a symbol. Each value is an integer number between 1 and the number of slots per frame. FIG. 1 illustrates a transmission sequence corresponding to one and the same symbol transmitted simultaneously by two separate channels. In the case of the example illustrated, the codes used for each channel are respectively 1, 3, . . . , 7 and 5, 4, . . . , 2.
Changing slot between two successive frames for one and the same channel, according to the code of the latter, is called time hopping. In addition to the multiplexing function which has been described, time hopping makes it possible to introduce redundancy, in order to improve the transmission quality. This redundancy corresponds to the multiplicity of the frames indicated by the code and which contain a particular symbol.
Furthermore, time hopping makes it possible to break up the periodicity of the transmitted radio signal. The energy of the radio signal is then distributed more continuously over all the frequency spectrum.
To identify a symbol transmitted by a transmitter to a predetermined receiver, the receiver must be synchronized relative to the transmitter. Once the synchronization is obtained, a symbol correlation pattern is generated within the receiver at the same time as the radio signal is received. This pattern is generated from the code of the channel used, and makes it possible to isolate, within the received signal, the pulses transmitted to the receiver. For this, the synchronization of the receiver must be performed with an accuracy of a few tens of picoseconds, typically.
Usually, the synchronization of the receiver is sought by implementing a rolling correlation between the pattern and the received radio signal. For this, correlation ratios between a sequence of the received signal and the pattern are calculated, by applying variable delay values to the pattern. The delay value associated with the highest correlation ratio obtained in this way corresponds to the synchronization. Given that the accuracy of the synchronization is then proportional to the number of delay values for which a correlation ratio is calculated, a considerable number of operations must be carried out within the receiver.
Such a synchronization method can therefore take a particularly long time, especially for low-rate transmissions. Now, the data that is contained in the signals received before the receiver is synchronized cannot be decoded. The result of this is that a large quantity of transmitted useful data is lost when setting up a new pulse radio connection.
Furthermore, the method of synchronization by rolling correlation requires the receiver to have major calculation means. In particular, several computer assemblies are usually arranged in parallel within the receiver, to reduce the time it takes to search for synchronization. The result is a complexity, a production cost and an energy consumption of the receiver that are not compatible with certain uses considered for pulse radio transmission.
Various refinements of synchronization by rolling correlation have been recently introduced, in order to reduce the duration of this operation. These refinements provide a faster convergence towards the value of the delay applied to the pattern which corresponds to the maximum of the correlation ratio. Despite these refinements, the synchronization of the receiver remains a lengthy and detrimental step.
Finally, synchronization by rolling correlation is incompatible with a receiver operating by energy detection. In practice, the amplitude of the radio signal must be identified by the receiver at each moment, even for very low signal levels, in order to calculate the correlation ratios between this amplitude and the correlation pattern.